1. Field of the Invention
This invention relates to hot runner injection molding systems and more specifically to externally heated injection runner nozzles for such systems.
2. Related Art
There is a need for a better way of heating injection nozzles in plastic molding machines.
Hot runner injection molding systems have several melted material flow passageways that are uniformly heated over the entire flow path leading from a molten reservoir to a mold cavity or cold runner. The melted material that flows through the passageway must remain liquid until reaching the mold cavity or cold runner. To control flow rate and pressure, the heated passageway leads to or from injection mold runner nozzles which may be externally heated. This nozzle is sometimes referred to as a hot runner gate injection nozzle or a hot runner probe injection nozzle but will hereafter be simply referred to as a "runner nozzle." These runner nozzles are typically located in the hot runner molding system's manifold base. The nozzles extend through ports leading to each of the molding cavities or to a secondary heated or unheated passageway within a mold block. It is essential to adequately and uniformly heat the runner nozzle because this is often the final point in the heated portion of the flow passageway just prior to being injected into the mold. At this point the material must be at or above its melting point in order for the molten material to flow freely through the runner nozzle, so the nozzle can reliably perform its function of controlling flow rate.
Significant transitions in temperature at the point of the runner nozzle are not desirable as the nozzle is a key part of any molding process because transitions in temperature may change the fluid consistency of the melted material such as thermoplastic which may result in a defective final product. Also, if it is desired to intermittently shut off flow and turn flow back on for a given nozzle, heating of the nozzle is necessary to maintain the residual material in a melted state, to prevent clogging.
Currently, runner nozzles are typically heated by a heat source external to the nozzle. Typically, the runner nozzle is heated by a resistive wire proportionally spirally wound heating element. The spirally wound element forms a cylinder that is co-axially disposed about the exterior surface of the runner nozzle. However this type of heater configuration operates inefficiently due to heat loss because of the open exposure of the heating element to the surrounding environment. It also increases the diameter of the nozzle and thus requires bigger openings in the manifold to receive the nozzle. Also, many of the standard nozzle heaters are not completely encapsulated by an insulated sheath, which make it more difficult to maintain a temperature at the runner nozzle location that is uniform with the remainder of the flow passageway. In addition the physical design of the resistive element (i.e. spiral) is limited as well. The gauge of the resistive wire heating element required to generate enough heat is such that the wire cannot be formed into complex circuit patterns. In many cases various complex circuit patterns other than a simple spiral pattern are desired in order to achieve more efficient heat distribution. Also, these types of heaters can be bulky and difficult to maintain and repair. Installation is difficult because of the large leads of the resistive element, and the mold designer must allocate space for the large leads and increased nozzle/heater combination. In addition, in many cases the externally heated runner nozzle apparatus has to be adapted to accommodate a thermocouple device which requires an additional space for the thermocouple and its wiring. A better way is needed to uniformly heat the runner nozzle, heat it efficiently and the design should be cost effective and easy to maintain and repair.
Conventional industrial equipment which provide heat externally to a flow passage, such as the subject runner nozzle, will generally provide heat by the means described above or by a single or multiple band heater design. However, there are some less common methods utilized to provide heat externally to a flow passageway. For example, one method is to apply a thick film resistive element layer to the external surface of a fluid flow passageway or to the external surface of a tubular heating sleeve by way of a decal application.
For the decal application process a resistive thick film pattern is printed on a sheet of silicone coated paper using standard pastes and screen print settings. The dried print is then covered with a clear acrylic coating, and the thick film decal is ready after drying. Soaking in water then releases the thick film paste with its acrylic top carrier film. This is then rolled onto the external surface of an object such as a tubular fluid flow passageway or a tubular heating sleeve. Firing in a conventional furnace burns off the acrylic layer and sinters the thick film pattern onto the surface.
The decal method is chosen by some artisans because they determined that printing a resistive trace circuit pattern on non-flat surfaces such as tubular surfaces proves very difficult or too expensive to be practical using standard screen printers because of the three-dimensional movements of the printer head which would be needed. A tubular heater sleeve bearing the decal application and co-axially disposed on the runner nozzle is a solution since it allows two-dimensional production of the circuit and subsequent bending of the decal in the third dimension. Also, the decal can be applied directly to the runner nozzle's external cylindrical surface. In either case a porcelain dielectric layer is usually applied to the metallic external surface of the tubular shaped object before the decal is applied.
However, the decal method has lots of problems. The decal method is not widely used because it is prone to error. Application of the thick film to the decal substrate is one step where errors may occur. Once the thick film is correctly two-dimensionally applied to the decal substrate, it has to be reapplied in the third dimension to a cylindrical external surface that has been properly prepped. The reapplication process may result in tears or voids in the thick film because at this stage the material is not a coherent solid mass. Air bubbles may form between the thick film and the final tubular surface. Firing in a furnace is required to cure and to burn off an acrylic layer. Again, a void may result from this process particularly if the acrylic burns off in a non-uniform manner.
In many cases it is desired to have multiple resistive element layers and multiple dielectric layers. If the decal application method described above is utilized to accomplish this task the problems asserted above will only be compounded. Proper alignment of each decal with immediately preceding decal is a significant problem. Either multiple decal application steps will have to be carefully and precisely performed or multiple layers will have to be initially applied to the decal prior to the transfer application step. However, in either case the problems asserted above such as voids and cracks will be multiplied both by the extra steps and extra layers. It should be noted that multiple firings will be necessary in order to cure each decal application layer thereby increasing the risk for a defective end product. This risk factor is important because multiple layers are often desired to achieve a certain temperature profile, or achieve greater thermal isolation, or to maintain temperature uniformity.
The decal method, as well as a screen printing method, have another serious drawback. They are limited as to the trace pattern that can be utilized for a heater design. In many cases, it is desired to print a continuous circumferential spiral trace pattern around a tubular body for efficient and uniform heat distribution. This is not physically achievable with a screen printing or decal printing method, which must have gaps for structural support in the mask and which will have edges that, for a tubular heater, would need to be so precisely aligned as to be impractical and would still have an interface or gap at the edges. Thus, neither a decal nor a screen can physically produce a continuous circumferential spiral trace pattern on a tubular body.
In addition to the decal method, another method is utilized specifically for hot runner nozzle external heaters. This method involves flat ceramic substrates and a printed circuit pattern thereon by means of screen printing. A printed circuit pattern is applied to a flat ceramic substrate sheet. When the printed circuit pattern is applied to the flat ceramic sheet, the sheet is in a paste or unfired state. Therefore the sheet is flexible and can then be wrapped around a tubular object. The sheet with the circuit pattern applied is usually wrapped around a ceramic core and fired for curing thereby completing the heating element. In known applications this completed heater can then be co-axially inserted onto a runner nozzle. This wrapping method could be used to wrap the ceramic substrate with the printed circuit applied around a metallic tube with a ceramic exterior surface thereby forming a tubular heating element. However, it cannot produce a continuous circumferential spiral trace pattern. It should be noted that problems similar to that of the decal method as discussed above may occur. Voids and tearing may occur during the wrapping and firing steps.
A way is needed to reliably manufacture a tubular heater with an optional continuous circumferential spiral trace pattern that can be co-axially disposed around or made an integral part of the exterior surface of an injection mold runner nozzle in order to externally heat the nozzle thereby replacing the conventional proportionally spirally wound resistive wire heating element. The method must be produceable, easy to accommodate and install, maintainable, and reliable.